ADAMs, a family of cell surface proteins containing a disintegrin and metalloprotease domains, play important roles in many biological processes involving cell surface proteolysis, cell-cell, or cell-matrix interactions. Our long-term goal is to understand the function of ADAM proteins and to dissect their roles in transmembrane signaling. Currently, we focus our studies on ADAM12, an ADAM family member that is involved in skeletal muscle development and/or regeneration. Our preliminary results suggest that ADAM12 plays a role in the induction of GO phase (quiescence) during myoblast differentiation in vitro. Quiescent, undifferentiated cells formed during myogenic differentiation in vitro share several characteristics with muscle satellite cells in vivo. Satellite cells play a pivotal role during muscle regeneration, muscle hypertrophy, and post-natal muscle growth, but the mechanism of self-renewal of the satellite cell compartment in skeletal muscle is poorly understood. The goal of this proposal is to understand the role of ADAM12 during GO entry in muscle cells. We hypothesize that the mechanism by which ADAM12 induces the entry into quiescence involves down-regulation of PI3K activity and depends on ADAM12-mediated cell-cell interactions. To test our hypothesis, we will perform a series of studies that will pursue the following Specific Aims. In Aim 1, we will characterize the molecular events associated with cell cycle arrest and the sequence of events leading to up-regulation of quiescent cell markers (retinoblastoma-related protein p130 and cell cycle inhibitor p27) by ADAM12 in myoblastic cell lines and in primary myoblasts. In Aim 2, we will perform a comprehensive analysis of the effect of ADAM12 binding on the activity of PI3K in cultured myoblasts. In Aim 3, we will examine the role of the extracellular sub-domains of ADAM12: disintegrin, cysteine-rich, and EGF-like region, in ADAM12-induced cell cycle arrest and up-regulation of p130 and p27. The results of our studies may help understand the biology of satellite cells and their role in muscle growth and repair.